1. Technical Field
The present invention relates to inter-system handover between two different radio access technologies (RATs) and, more particularly, an inter-system handover accomplished by a dual-system mobile phone capable of communicating with both systems without needing a dual receiver and without needing to use compression/special gaps created in transmission of the originating system to allow measurement of the target system during breaks.
2. Discussion of Related Art
In order to be able to interoperate between mobile telecommunications systems using distinct radio access technologies (RATs) it is necessary to have a dual-system mobile phone capable of communicating with both RATs. Before a handover between two RATs can be initiated the quality of the signal from a new cell in the target RAT has to be measured and compared to the old cell. Doing this measurement requires special arrangements in systems that use continuous transmission and reception on dedicated channels. One prior art approach to solving the problem of how to make measurements on the target RAT while at the same time receiving continuously in the originating RAT is to provide two receivers in the mobile station so that one receiver can be receiving a signal from the target RAT for evaluation in parallel with the continuous reception from the originating RAT. In this way, an active transmission of audio in a speech call for instance can be ongoing while the mobile phone is at the same time receiving and evaluating a signal from the candidate target RAT. Another prior art alternative is to create gaps in the downlink of the originating RAT using compression techniques so as to allow measurements of the target RAT during the gaps. Implementing dual receivers increases terminal cost and to some extent also power consumption. Compressed mode allows only limited length measurement gaps and forces a rise in transmission power of the compressed part to maintain the original transmission data rate. In some cases the data rate may need to be dropped to prevent exceeding the terminal maximum power or creating unsustainable interference to neighbors in the originating system.
An objective of the present invention is to facilitate inter-system handover while avoiding the above-mentioned disadvantages of the prior art.
According to a first aspect of the present invention, inter-system handover between two mobile telecommunications systems using distinct radio access technologies is facilitated by transmitting a pilot signal from a base station of a first radio access technology (RAT) used in a first mobile telecommunications system on a carrier having a frequency used by a second RAT used in a second mobile telecommunications system, and by receiving the pilot signal in a dual-system mobile station capable of operating in both the first mobile telecommunications system using the first RAT and in the second mobile telecommunications system using the second RAT for evaluating the pilot signal for determining whether to carry out an inter-system handover from operating the mobile station in the second mobile telecommunications system using the second RAT to operating the mobile station in the first telecommunications system using the first RAT.
In further accord with the first aspect of the present invention, the dual-system mobile station, in furtherance of the determination of whether to carry out an inter-system handover, measures the signal strength of the pilot signal. Further, it may measure the signal strength of the downlink from the second RAT. A power ratio may furthermore be determined between the pilot signal and the carrier of the second RAT. This information may be used in the mobile station in determining whether to efect an inter-system handover or may be communicated on an uplink of the second RAT to the second telecommunications system for making an inter-system handover decision at the network level. If such a determination is made at the network level it will be communicated on a downlink back to the dual-RAT mobile station.
In still further accord with the first aspect of the present invention, the pilot signal communicated from the first RAT to the dual-RAT mobile station may include synchronization information for enabling the dual-RAT mobile station to synchronize with the first RAT in the event of an inter-RAT handover decision.
According to a second aspect of the present invention, a dual-system mobile station capable of operating in both a first mobile telecommunications system using a first radio access technology (RAT) and in a second mobile telecommunications system using a second RAT comprises a receiver for receiving a pilot signal for providing a signal of a magnitude indicative thereof; and a pilot signal evaluator, responsive to the pilot signal from the receiver for evaluating the pilot signal for determining whether to carry out an inter-system handover from operating the mobile station in the second mobile telecommunications system using the second RAT to operating the mobile station in the first telecommunications system using the first RAT and for providing a signal indicative thereof.
In further accord with the second aspect of the present invention, the signal provided by the evaluator is indicative of a measured signal strength of the pilot signal and wherein the mobile station further comprises a determiner for comparing the pilot signal strength to the signal strength of a carrier of the second mobile telecommunications system using the second RAT, for providing a power ratio signal for transmission to the second mobile telecommunications system using the second RAT, for use by said second mobile telecommunications system using the second RAT in deciding whether to carry out the inter-system handover.
In still further accord with the second aspect of the present invention, the mobile station further comprises an evaluator responsive to a signal from the receiver having a magnitude indicative of the strength of a broadcast signal from the second mobile telecommunications system using the second RAT for providing a signal having a magnitude indicative of said signal strength of said broadcast signal.
According to a third aspect of the present invention, a base station of a second generation mobile telecommunication system having an antenna for radiating signals to mobile stations in a cell thereof is characterized by a pilot signal transmitter for transmitting a pilot signal having a carrier frequency identical to that of a radio access technology of a third generation mobile telecommunication system for conveying information relating to its identity and the configuration parameters needed by a dual-system mobile station capable of operating in both said second and third mobile telecommunications systems for use in making a handover decision to change from operating in said third generation mobile telecommunication system to operating in said second generation mobile telecommunications system.
It will be appreciated that measurements can be made by the original system of the target RAT to determine the pilot strength and synchronization of the target system by receiving the special transmission of the present invention on the channel of the original system. In this way costly dual receivers are avoided in the mobile station or the alternative compression of frame transmission is to allow measurement gaps is avoided.
Network-related disadvantages would however include adding the necessary means to transmit for instance a GSM pilot and synchronization information on a WCDMA carrier, the need to co-site the for instance GSM cells with the cells from which the measurements are made (e.g., WCDMA), and the disadvantage of areas around, e.g., the GSM cells becoming xe2x80x9cshadow regionsxe2x80x9d when the GSM pilot on the WCDMA band blocks the reception of the WCDMA cell. Another disadvantage is the need to standardize this concept.
Although a specific example is shown relating to GSM/WCDMA, it should be realized that the present invention is broadly applicable to inter-system handover between second, third, fourth, etc. generation radio access technologies.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as illustrated in the accompanying drawing.